Article pubs.acs.org/EF
Process Simulation and Validation of Chemical-Looping with Oxygen Uncoupling (CLOU) Process using Cu-Based Oxygen Carrier Ling Zhou,†,‡ Zheming Zhang,† Chris Chivetta,† and Ramesh Agarwal*,† †
Department of Mechanical Engineering & Materials Science, Washington University, St. Louis, Missouri 63130, United States Research Center of Fluid Machinery Engineering & Technology, Jiangsu University, Zhenjiang 212013, China
‡
ABSTRACT: Several experiments have demonstrated that the chemical-looping with oxygen uncoupling (CLOU) provides an effective technological pathway for high-efficiency low-cost carbon dioxide capture when particulate coal serves as the fuel. In this paper, complete process-level modeling of CLOU process is established in ASPEN Plus based on a series of detailed experiments. The heat content of fuel and air reactors and air/flue gas heat exchangers is carefully examined. It is shown that the established model provides results that are in excellent agreement with the experiments for the flue stream contents of both the reactors, oxygen carrier conversion kinetics, and the overall performance of the CLOU process. Four scaled-up cases are also carried out to investigate the influence of increase in the coal and oxygen carriers feeding. Additionally, three different types of coal as the solid fuel are also investigated to determine their effect on the CO2 concentration in the flue stream and on the overall energy performance.
1. INTRODUCTION Chemical-looping combustion (CLC) process is currently considered as a leading technology and one of the best alternatives for reducing the economic cost of CO2 capture;1 it requires much less energy for CO2 capture compared to other CO2 capture processes.2 CLC employs a dual fluidized bed system (circulating fluidized bed process) where an oxygen carrier (OC) is used as a bed material providing the oxygen for combustion in the fuel reactor. The reduced OC is then transferred to a second bed and is reoxidized in an air reactor before returning back to the fuel reactor. Theoretically, the heat release in the fuel reactor and the air reactor is the same as in the conventional combustion. 3,4 However, due to the elimination of gas separation process, the only energy loss associated with CLC process is due to the relatively insignificant pressure loss which drives the pneumatic circulation of OCs within the system. When solid fuel (e.g., coal) is utilized by a CLC system, it is often the case that the reactivity of char is low; it is due to the limited contact of oxygen carrier and gasified coal. An alternative process known as the chemical-looping with oxygen uncoupling (CLOU) has recently been proposed to overcome the low reactivity of the char gasification stage in the direct coal chemical-looping combustion.5−8 CLOU is based on the strategy of using materials that release gaseous oxygen in the fuel reactor as the oxygen-carrier thereby allowing the solid fuel to burn with gas phase oxygen. These materials can also be regenerated in high temperature environment, for example in the air reactor. Thus, in CLOU, the slow gasification step in the chemical-looping combustion with solid fuels is avoided, resulting in a much faster solid conversion.6 In a CLOU process,9−12 CO2 and H2O are produced in the fuel reactor by different reactions. The selection of an OC that can release oxygen in the fuel reactor and reverses back to original state after reoxidization in the air reactor is limited by the reactor temperature. Only those OCs that have a suitable © 2013 American Chemical Society
equilibrium partial pressure of gas phase oxygen at temperatures of interest for combustion (usually ranging from 800 to 1200 °C) can be used for the CLOU process.13 So far, three materials have been identified as suitable OCs: CuO/Cu2O, Mn2O3/Mn3O4, and Co3O4/CoO.5 In this paper, a CLOU process utilizing copper(II) oxide as the oxygen carrier is considered; the oxygen is released according to the reversible reaction: 4CuO ⇄ 2Cu 2O + O2
(1)
The coal fed into the fuel reactor undergoes a two stage process. It first devolatilizes, producing a solid residue char and volatile matter as gas product: coal → volatile matter + char
(2)
Then, the char and the volatiles are burnt as in a standard combustion process according to the following reactions: char (mainly C) + O2 → CO2
(3)
volatile matter (mainly H 2 , CO, CH4) + O2 → CO2 + H 2O
(4)
Furthermore, the reaction of the copper-based OC with carbon (which is an exothermic reaction) can be accomplished at a lower temperature in the fuel reactor, as given: C + 4CuO → 2Cu 2O + CO2
(5)
After steam condensation, a pure CO2 stream is obtained from the flue gas of the fuel reactor. The reduced oxygen carrier is transported to the air reactor, where the oxygen carrier is reoxidized by the reverse reaction with the oxygen in the air, and then, it becomes ready for a new cycle.14−16 The flue gas Received: August 16, 2013 Revised: October 10, 2013 Published: October 15, 2013 6906
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Figure 1. Schematic view of the apparatus used in Abad et al.’s experiment.4
Table 1. Properties of Bituminous Colombian Coal “El Cerrejon” proximate anal. (wt %)
ultimate anal. (wt %)
energy
components
moisture
volatile matter
fixed carbon
ash
C
H
N
S
O
ash
LHV (kJ/kg)
fresh pretreated
7.5 2.3
34.0 33.0
49.9 55.9
8.6 8.8
70.8 65.8
3.9 3.3
1.7 1.6
0.5 0.6
7.20 17.6
15.9 11.1
25880 21899
to the fuel reactor, and a valve to control the circulation of solid flow rate in the system. To the authors’ knowledge, this experiment is the first time that the CLOU process has been demonstrated in an experiment utilizing two interconnected fluidized-bed reactors using a solid fuel. The solid fuel used in the experiment is a bituminous Colombian coal “El Cerrejon”.4 It should be noted that the coal is subjected to a thermal pretreatment for preoxidation in order to avoid coal swelling and bed agglomeration. Coal was heated at 180 °C in the atmospheric air for 28 h. Proximate and ultimate analyses of the pretreated coal are given in Table 1. Both the experiments and the ASPEN Plus simulations are based on this pretreated coal. The coal particle size used for this study is 200 to 300 μm. Oxygen carrier particles are prepared by spray drying, containing 60 wt % CuO and use 40 wt % MgAl2O4 as supporting material. The inclusion of supporting material is to increase the reactivity, durability, and fluidizability of the oxygen carrier.22 The particle size of the oxygen carrier varies between 100 to 200 μm. The effect of operating conditions on the combustion and CO2 capture efficiencies are investigated. The oxygen carrier decomposes in the fuel reactor, exhausting gaseous oxygen to the surroundings. The oxygen burns the volatiles and char produced from coal pyrolysis in the
stream from the air reactor contains only N2 and unreacted O2. Therefore, CLOU technology has a low energy penalty for CO2 separation and results in low CO2 capture costs. The heat released in the fuel reactor and in the air reactor is the same as in case of conventional combustion.8,17−21 In the present study, we conduct a detailed process simulation of the CLOU process and demonstrate the ability of ASPEN Plus code to accurately simulate the observations and results in the recent experiments. The results of simulations are compared with the experimental data for quantities such as concentration of various gaseous components in the flue stream and the energy released from both the air reactor and the fuel reactor.
2. BRIEF DESCRIPTION OF THE CLOU EXPERIMENTAL APPARATUS AND RESULTS (ABAD ET AL.4) A CLOU test apparatus, directly utilizing solid coal as the fuel, with a 1.5 kWth output was recently built by Abad et al.4 A schematic of the CLOU apparatus is shown in Figure 1.The experimental setup basically consists of two interconnected fluidized-bed reactors joined by a loop seal, a cyclone for gas− solid separation for transport of only solid from the air reactor 6907
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As summarized in Table 3, in ASPEN Plus coal devolatilization is defined by the RYIELD reactor, followed
fuel reactor. The reoxidation of the oxygen carrier takes place in the air reactor, consisting of a bubbling fluidized bed followed by a riser. N2 and unreacted O2 leave the air reactor passing through a high-efficiency cyclone and a filter before the stack. In Abad et al.’s experiment,4 a series of tests were conducted using the same oxygen-carrier. From the experiments, the series of tests with different coal feeding mass were selected for validation of the process simulation in ASPEN Plus. Table 2 summarizes the details of the experimental operational parameters and the results obtained from the experiments.
Table 3. Process Models Used in Different Parts of CLOU Process in ASPEN Plus
Table 2. Operational Parameters Used in Abad et al.’s Experiment4 test no.
TFR (°C)
φ
λ
ṁ s (kg/h)
ṁ coal (g/h)
power (W)
CLOU1 CLOU2 CLOU3 CLOU4 CLOU5
924 929 917 920 925
4.3 3.2 2.6 2.1 1.1
4.7 3.5 2.8 2.3 1.2
9.0 9.0 9.0 9.0 9.0
67 89 112 135 256
410 541 681 821 1560
3. PROCESS SIMULATIONS IN ASPEN PLUS ASPEN Plus is the well-known software widely used for systemlevel chemical process simulation. It can be used for industrial scale process simulation of an entire plant making it possible to predict the steady-state behavior of a full-scale petrochemical plant. For the study of CLOU process, ASPEN Plus can be employed for designing and sizing the reactors, for predicting the reaction conversion efficiency, and for understanding the reaction equilibrium behavior. For the Abad et al.’s experimental apparatus,4 we establish an ASPEN Plus model shown in Figure 2.
name
model
function
reaction formula
DECOMP
RYIELD
BURN
RGIBBS
FUEL-R
RSTOIC
coal → volatile matter + char char + volatile matter + O2 → CO2 + H2O 4CuO → 2Cu2O + O2
AIR-R
RSTOIC
SEP-F
SSPLIT
SEP-A
SSPLIT
SEP-B
SSPLIT
COOL-F
HEATER
COOL-A
HEATER
coal devolatilization and gasification syngas and char burn with O2 carrier reduction reaction carrier oxidation reaction O2 and Cu2O separation CuO and air separation separation, ash and flue gas flue gas cooler, fuel reactor flue gas cooler, air reactor
2Cu2O + O2 → 4CuO ∼
H2O(gas) → H2O(liquid) ∼
by the gasification of coal represented by the RGIBBS reactor. The RSTOIC reactor defines the actual fuel combustion. It should be noted here that the three reactor blocks together represent the fuel reactor in Abad et al.’s experiments.4 The flow sheet within the ASPEN Plus simulation package cannot model this entire reaction with one reactor. As a result, the fuel reactor is broken down into several different reactor simulations. The air reactor is modeled as an RSTOIC reactor. The molar flow rate of CuO exiting and Cu2O feeding in two separate blocks is defined to be identical to represent the
Figure 2. Overall flow sheet of CLOU process in ASPEN Plus. 6908
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Table 4. Values of Input for Mass Basis Yield into the DECOMP RYIELD Reactor component
H2
O2
N2
H2O
S
C
ash
mass basis yield
0.0322
0.1720
0.0156
0.0230
0.0059
0.6278
0.1235
Cu2O) is transported to the air reactor for reoxidization. The air reactor is represented by AIR-R RSTOIC reactor. Hot air with temperature of 200 °C is fed into the reactor at a rate of 1980 L/h. CO 2 , H 2 O, O 2 , O 2 S, NO, NO 2 , and N 2 concentration are monitored at the fuel reactor flue stream of CO2, and O2 concentration is measured at the air reactor fuel stream of N2−O2. Dry CO2 and H2O concentration are evaluated by eq 14.
circulation of OC within the system; such circulation cannot be defined explicitly in the ASPEN Plus modeling. 3.1. Model Setup for DECOMP RYIELD Reactor. For each unit feeding of coal, the DECOMP RYIELD reactor has a certain value corresponding to the component yield. In order to represent the decomposition of coal within the DECOMP RYIELD reactor, it is required that a breakdown of mass basis should be calculated in the manner shown in eqs 6−12, where Xi is the percentage breakdown within the coal for component i. Basis yield H 2 , mH2 = (1 − XMoisture)*XH
(6)
Basis yield O2 , mO2 = (1 − XMoisture)*X O
(7)
Basis yield N2 , mN2 = (1 − XMoisture)*XN
(8)
Basis yield H 2O, mH2O = XMoisture
(9)
Basis yield S, mS = (1 − XMoisture)*X S
(10)
Basis yield C, mC = (1 − XMoisture)*X C
(11)
Basis yield ASH, mASH = (1 − XMoisture)*XASH
(12)
CO2,Dry =
MCuO(4ΦΩcoalmcoal ) 0.6
(1 − H 2OFlue )
O2,Dry =
O2,Flue (1 − H 2OFlue )
(14)
In eq 14, CO2,Flue and H2OFlue are the fractional concentration of gases at the outlet flue stream of the reactors. Dry concentration is used for a more pure CO2 stream without water. Impurities in CO2 streams could have an effect on their sequestration behavior. Besides, power output is monitored at Q-Burn, Q-A, Q-C-A, and Q-C-F, as locations indicated in the model flow sheet in Figure 2.
4. RESULTS AND DISCUSSION 4.1. Gas Concentration. Figure 3 compares the CO2 and O2 concentration in the fuel reactor flue stream, which were
The coal does not have 100% char conversion efficiency. Abad et al.4 have measured the char conversion efficiency and found that the conversion efficiency varies slightly for different coal feeding rates. An approximate value of 97.66% char conversion is calculated from the average char burn and is considered in the modeling. Coal with properties defined in Table 1 is fed into the RYIELD reactor at five different feeding rates specified in five experimental tests, CLOU1− 5, as summarized in Table 2. The mass basis yield inputs for the RYIELD reactor are summarized in Table 4. 3.2. Model Setup for OC Circulation. Equation 13 given below is used to calculate the circulation rate of the OC within the fuel reactor. Denoting FCuO60 as the mass flow rate of OC solids (60 wt % CuO supported by 40% MgAl2O4) and MCuO as the molecular weight of CuO, a value of Φ is then defined to calculate the stoichiometric solid circulation rate needed for combustion for a given coal feeding rate. A value of Φ = 1 corresponds to the stoichiometric flow of CuO to fully convert coal to CO2 and H2O via reactions given by eqs 1−5, where the CuO is reduced to Cu2O. In eq 13 below, mcoal is the mass flow rate of coal feed into the fuel reactor, and Ωcoal is the stoichiometric number of moles of oxygen needed to convert one kilogram of coal into CO2, H2O, NO, and SO2. The value of Ωcoal can be calculated from the proximate and ultimate analysis of coal, yielding a value of 59 mols of O2 per kilogram of coal. FCuO60 =
CO2,Flue
Figure 3. CO2 and O2 concentration at fuel reactor outlet.
obtained in both the experiments and simulations. Obviously, the CO2 and O2 concentrations increase almost linearly with increase in the coal feeding rate. The deviation from linearity at low coal feeding rate is probably due to the incomplete mixing of OC and coal, which is significantly mitigated at higher coal feeding rate. The relatively low concentration of O2 in the flue stream, as shown in Figure 3, suggests that the decoupling of oxygen from the OC occurs as an on-demand fashion; that is, it is determined by the presence of coal in the system. Figure 4 shows that the depletion of O2 in the air reactor is increasing with increase in the coal feeding rate. Overall, CO2 and O2 concentration levels predicted by the ASPEN Plus are in good agreement with the experimental data. Possible improvements in simulation can be obtained by calculating a more accurate FCuO60 value and char combustion value.
(13)
For modeling, the average FCuO60 value of 8.94 kg/h is used for the five test cases. N2 is fed into the fuel reactor for fluidization. The fluidization agent does not have any influence on the oxygen uncoupling behavior of a Cu-based oxygen-carrier.4 3.3. Model Setup for AIR-R RSTOIC Reactor. After releasing O2, reduced OC solids (with active component 6909
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have a linear relation with respect to coal feeding rate due to variation of air flow within the air reactor. It is also important to obtain the conversion characteristics of the CLOU system. Therefore, the CO2 and O2 concentrations in the fuel reactor flue stream and the O2 concentration in the air reactor flue stream are examined. 4.3. Energy Requirements. Figure 6 compares the thermal power output for the entire CLOU system. It can be seen that
Figure 4. O2 concentration at fuel reactor outlet.
4.2. Oxygen Carrier Efficiency. One of the most important indicators of the performance of a CLOU system is the oxygen carrier efficiency, ΔOC. The ΔOC is defined as the fraction of oxygen carrier conversion of CuO into Cu2O and O2; it is the ratio of transferable oxygen in the current oxygen carrier to that in the fully oxidized oxygen carrier as given by eq 15.
Figure 6. Overall power comparison between the simulation and the experiment.
⎧⎛ ⎪ ΔOC = ⎨⎜⎜FCO2,out,FR + FO2,out,FR + 0.5(FCO,out,FR ⎪⎝ ⎩
the estimated power output given by ASPEN Plus model is in reasonably good agreement with the experimental values. The discrepancy between the simulation and the experiment could be attributed to slightly different working conditions in the experimental apparatus. Inevitable losses at multiple locations of the experimental apparatus are expected, which, however, are not considered in the process modeling. Table 7 summarizes the breakdown of power output for various components of the modeled CLOU system. It should be noted that, for thermal power output, the negative value indicates thermal input rather than output. Because the thermal energy associated with coal decomposition and fuel circulation stays within the system rather than being extracted for power generation, it does not contribute to the thermal output and therefore is excluded from the evaluation of the CLOU system’s power output. 4.4. Scaled-up Simulations. Scaling up is an essential step and an important task for the realization and optimization of commercial plants. Four scaled-up cases were conducted for the test case CLOU5. In these cases, the coal feeding rate is scaled up by a factor of up to 5. The OC solid circulation rate, air supply rate is also scaled up correspondingly to meet the demand of the increased coal feeding. Other modeling parameters such as the reactor efficiency and coal decomposition rate remain unchanged for the scaled up simulation of CLOU5. The total thermal power output and main gas compositions for the flue stream from the air reactor and the fuel reactor are summarized in Table 8. It can be seen that the total power output increases linearly with increase in coal feeding; however, the flue stream composition remains very steady for all the scaled up cases. Considering the principles of energy and mass balance on which ASPEN Plus modeling is based, linearity in the scaled-up results is expected, since the nonlinear effects (e.g., the energy loss at multiple locations) are inherently omitted in the modeling process.
⎫ ⎛⎛ f ⎞ ⎛ f ⎞⎞⎞ ⎪ HO + FH2O,out,FR ) − 0.5mcoal ⎜⎜⎜⎜ 2 ⎟⎟ + ⎜ O ⎟⎟⎟⎟⎟MO2 ⎬ ⎪ ⎝⎝ M H 2 O ⎠ ⎝ M O ⎠⎠⎠ ⎭ /{0.25FCuO}
(15)
In eq 15, Fx is the concentration of dry gas ‘x’ in the output flue stream of the fuel reactor, mcoal is the coal feeding rate, f i is the mass fraction of coal in the compound i, and Mx is the molecular weight of compound x. As shown in Figure 5, ΔOC varies linearly with the coal feeding rate. This implies the abundance of OC in the system. However, the air reactor efficiency, defined as the fractional oxygen carrier conversion of Cu2O and O2 into CuO, does not
Figure 5. Oxygen carrier efficiency comparison between the simulation and the experiment. 6910
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Table 7. Thermal Analysis of Various Locations of the Modeled CLOU System test no.
total power (W)
Q-A (W)
Q-Burn (W)
Q-C-A (W)
Q-C-F (W)
Q-Decomp (W)
Q-F (W)
CLOU1 CLOU2 CLOU3 CLOU4 CLOU5
436.6 606.4 777.6 946.5 1591.4
−175.1 −79.9 −30.5 51.5 180.3
116.4 181.9 296.1 372.7 803.6
380 370.1 361.1 352.3 338.2
115.3 134.3 150.8 170 269.3
31.6 41.7 53.5 64.2 120.7
−380.1 −477.6 −534.5 −628.8 −1094
Table 8. Scaled-up Simulations Based on Test Case CLOU5 input
output
test no.
ms (kg/h)
mcoal (g/h)
vn2 (LN/h)
vair (LN/h)
power (W)
CO2,FR (vol. %)
O2,FR (vol. %)
O2,AR (vol. %)
CLOU5 CLOU5-1 CLOU5-2 CLOU5-3 CLOU5-4
9 18 27 36 45
256 512 768 1024 1280
186 372 558 744 930
1980 3960 5940 7920 9900
1591.4 3182.8 4774.2 6365.6 7957
62.67 62.67 62.67 62.67 62.67
0.69 0.69 0.69 0.69 0.69
4.95 4.95 4.95 4.95 4.95
Table 9. Properties of Three Types of Coals proximate analysis (wt %)
ultimate analysis (wt %)
energy
coal name
moisture
volatile matter
fixed carbon
ash
C
H
N
S
O
ash
LHV (kJ/kg)
bituminous anthracite lignite
2.3 1.0 12.6
33.0 7.5 28.6
55.9 59.9 33.6
8.8 31.6 25.2
65.8 60.7 45.4
3.3 2.1 2.5
1.6 0.9 0.6
0.6 1.3 5.2
17.6 2.4 8.5
11.1 32.6 37.8
21899 21900 16250
4.4. Simulations of CLOU with different types of coal. It is also important to test the performance of a CLOU system for various types of coal. Three different types of coal are used with the Cu60AlMg oxygen carrier; these are bituminous Colombian coal, “El Cerrejon” anthracite coal from El Bierzo, and lignite coal from Teruel basin.4 The selections of these coal samples are based on the consideration of diversity in their volatile matter content and carbon content, since it is of great interest to investigate the performance of CLOU process over a ranger of coal properties. Therefore, samples from three different coal ranks, bituminous, anthracitic, and lignite are considered in this modeling. The detailed properties of these coals are summarized in Table 9. The process simulations with three different types of coal are conducted with different coal feeding rates; the detailed operational parameters are set to be the same as given in Table 2. The CO2 concentration in the flue stream of the fuel reactor outlet and the overall energy released are compared in Figure 7 and 8, respectively. For all three types of coal, the flue stream CO2 concentrations and the power outputs increase with the higher coal feeding rate as well as with the higher carbon content in the coal. Since the carbon content of bituminous coal and anthracite coal is similar, the results for these two types of coal show similar characteristics for the flue stream CO2 concentration and the overall power output. In contrast, the lowest flue stream CO2 concentration and least power output are obtained for the lignite coal due to its low carbon content. Overall, characteristics of the flue stream CO2 concentration and power output for different types of coal have similar increasing trend with higher coal feeding rate. However, the performance of the CLOU process depends greatly on the coal reactivity, which is mainly related to the coal rank. The high grade coal can produce CO2 of almost twice the purity compared to the low grade one. This suggests that a more active oxygen carrier is needed when low rank coal is utilized as fuel for a CLOU process.
Figure 7. Flue stream CO2 concentration at fuel reactor outlet for three different types of coal.
5. CONCLUSIONS In this study, ASPEN Plus is employed to model and study the CLOU process. The ASPEN Plus simulations are established using information from a series of test cases conducted in a CLOU experiment. Excellent agreement is obtained between the experimental and simulation results for various quantities such as the oxygen carrier conversion kinetics, flue stream O2, and CO2 concentrations, and power output. It is clearly demonstrated that the ASPEN Plus can provide a creditable process simulation platform for the study of CLOU process. Scaled-up simulations are also conducted using different types of coal and coal feeding rates. The results show that the total power output is nearly linear with the increase in coal feeding rate and carrier circulation. Such linearity is, in general, not expected for actual scale-up, since the ASPEN Plus system modeling software neglects miscellaneous energy losses in the 6911
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Figure 8. Overall energy released in CLOU process for three different types of coal.
system due to changes in the hydrodynamic characteristics. To account for the changes in the hydrodynamics characteristics, detailed hydrodynamic simulations are needed using the computational fluid dynamics software. Furthermore, the coal rank appears to have significant impact on the flue stream CO2 concentration and overall energy release; the bituminous coal and anthracitic coal show similar improved CLOU performance compared to the lignite coal. The similarity in the CLOU performance of bituminous coal and anthracitic coal can be explained by the fact that they have similar carbon content. The results indicate that char gasification is no longer a performance factor of high relevance since the presence of oxygen enables the solid−gas combustion to take place without gasification.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was partially supported by the Consortium for Clean Coal Utilization (CCCU) at Washington University in St. Louis, the National Studying Abroad Foundation of China and Missouri STARS program.
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REFERENCES
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